When I read a history book, I like to read reportage, not editorialization.
When i began research for The Ensign Locker, one of the things I did was to read all the St. Louis Post Dispatch newspapers from 1965 and 1966. I had carried the impression away from those years that the press didn't give the military a fair shake in reporting the war. After reading two years worth of newspapers, I was surprised to find that the reporting was good, factual, and didn't appear to me to be biased. The editorial pages, however, I didn't care for.
That's what I prefer in the history books I read. A writer can't help but slip a little editorialization in now and then, probably. I remember a Halberstam book, I'm pretty sure it was The Coldest Winter. I was about 80% finished with the book, and appreciating it, when he sticks in a short paragraph of what I'd characterize as editorializiing. Then he went back to reporting. I've wondered whether he put that in just to see if the reader was still paying attention. Whatever his purpose, it was cleverly done. In that book, the beginning of chapter 53 has as good a summary of world events in the late forties and fifties as any I've seen. Reading a little editorializing to get to that was a very small price to pay.
One piece of history book editorializing I have appreciated is the note to the reader in the beginning of volume 7 of the Durant's Story of Civilization. One sentence in the second paragraph of the note talks about "the rise of murderous nationalism and the decline of murderous theologies." I'd never thought of the age of reason that way before and appreciated the suggestion.
At any rate, ten editorial words amongst 1,000,000 or so words of reportage, that's the right balance